The present invention relates to methods of joining and uniting objects made of weldable thermoplastic polymers. In particular, the invention is concerned with weldingly connecting two or more self-supporting structures, such as tubes, pipes, sleeves, fittings and the like articles made of weldable thermoplastics in an area of mutual overlap by heating a limited area so as to produce mutual fusion and coalescence in the welding area and allowing the weld to cool and solidify for producing an integral connection joining the welded structures.
Such methods are well known in the art and have been defined in various standards, e.g., German Industrial Standards DIN 1930, Sheet No. 3, "Welding of Plastics". Specific methods and applications are disclosed, for example, in Swiss Pat. Nos. 336,980, 349,404 and 396,536, in German Patent DT No. 802,282, the published German Applications DT-AS Nos. 1,071,433, 1,099,157 and 1,704,318.
All prior art methods for welding thermoplastic structures of the type mentioned are intended to produce homogeneous fusion in the welding zone, i.e. where the parts of the objects to be joined are subjected to conditions sufficient to transform the normally solid thermoplastic polymer composition by the impact of heat, and preferably by the additional impact of pressure, into a state capable of coalescence; generally such state is reached when the materials to be welded are at least plastic.
Hereinbelow, the terms "thermoplastic polymer compositions" and "thermoplastics" are used interchangeably to broadly refer to those synthetic structural materials or plastics which can be said to be macromolecular or polymeric (having appropriate molecular weights of, typically, from a few thousand to several millions) and which are obtained by various methods including polymerization by polyaddition, polycondensation and the like methods of converting so-called "mers" (monomers or low molecular polymers) into high molecular products. Specific examples of suitable thermoplastics will be given below but it is to be understood that this term is to include homopolymers, heteropolymers (polymers produced from two, three or more different mers by simultaneous or subsequent reaction) as such or in mixtures with each other or with conventional additives, adjuvants, fillers, etc., the essential common denominator of thermoplastics being their capability of reversible thermal conversion into an at least plastic state by heating the normally solid thermoplastics. The term "at least plastic" is to be understood here as referring to any flowable state of thermoplastic polymer compositions (with or without additions) which under the conditions of the welding method used results in a sufficient interaction of the plastified bonding zone for mutual "welding" together, i.e. thermal fusion, of the welded parts, and which may range from thickly viscous to liquid, depending upon the thermoplastics involved (molecular weight, melt index, linearity, effect of additions). For example, welding by thermal fusion may be achieved with given thermoplastics even before actual fluidity is reached, e.g. by pressing engagement of thermoplastics in a heat-softened or plastic state.
The temperature, which for a given thermoplastics mass under the operating conditions including the pressure, mechanical action and the like, leads to formation of a permanent and integral bond, is designated hereinafter as "welding temperature".
The need for applying a pressure at the welding location in thermoplastics welding is an accepted concept in many standards (e.g. DIN 1930). The magnitude of the pressure actually used in thermoplastics welding does vary widely however, as do the conceptual models of the behaviour of the macromolecule chains of thermoplastics in the region of the welding zone, i.e. the bonding zone in an at least plastic state.
The conventional concept of a mutual interflow upon welding with concomitant interlocking or tangling of the polymer molecules in the bond region of welded parts of thermoplastics has recently been challenged by the hypothesis of a predominantly adhesive bond of "subparticles", i.e. molecular aggregations. On account of the wide differences between the various compositions summarized under the term thermoplastics, in regard to composition and molecular structure, it is questionable whether a single theory of thermoplastics welding is possible for all thermoplastics known to be capable of welding. However, all serious prior investigations of the relation between strength and structure of weld connections indicated that the general aim was to achieve weld zones which are as homogeneous and notch-free as possible. This implies several limitations as to operating conditions and the geometry in the welding area, i.e. the requirement of avoiding gaps at the interfaces.